It is known to clean flue gases both of solid particles and of nitrogen oxides and sulphur dioxide, and it is known to utilize the heat of the flue gas. The previously known plants ensure a reduction of about 90% of the amount of sulphur dioxide in the flue gas. The remaining amount of sulphur dioxide assures that the flue gas discharged into the atmospheric air is particularly aggressive, said flue gas usually being discharged at a temperature of about 60.degree.-70.degree. C.